A typical mass flow controller (MFC) is a device that sets, measures, and controls the flow of a fluid gas in industrial processes such as thermal and dry etching among other processes. An important part of an MFC is a thermal flow sensor that measures the mass flow rate of the gas flowing through the device. While the thermal flow sensor is measuring actual flow, the indication of the measured flow is reported to the user/operator as an “indicated flow” via an indicated flow output. In many instances, however, it is not desirable to report the actual signal from the thermal sensor as the indicated flow to the user/operator of the MFC because the actual signal from the thermal sensor may misrepresent the actual fluid flow and may produce false alarms.
The thermal flow sensor of a mass flow controller (MFC) generally produces a very slow signal even when the actual flow is changing fast. Because the response time of the sensor is critical for stable control of the fluid flow, the slow signal from the thermal flow sensor is typically accelerated by processing it with “acceleration” filters. This type of acceleration, however, also increases the signal noise, but this “acceleration noise” does not reflect the actual noise of fluid flow running through the sensor—it is just a side effect of making the signal faster. Although the acceleration noise does not affect the algorithm that controls the flow rate, it can give the wrong impression to an operator of the MFC about the quality of actual fluid flow. Even worse, the acceleration noise can trigger a false alarm on the processing tool. As a consequence, to avoid false alarms, the acceleration noise should be reduced as much as possible when reporting flow to the MFC operator.
Due to the design of MFCs, during some conditions there may be some “internal” flow in the MFC that is undesirable to report to an operator of the MFC. This internal flow does not go to the output of MFC (to the processing tool), but it may pass through the flow sensor, and it may be reported to the operator as an actual flow delivered to the processing tool when in reality the sensed internal flow is not delivered to the processing tool. This internal flow may happen, for example, when inlet pressure changes, and some amount of fluid fills in a “dead volume” (the volume between the flow sensor and valve) to equalize the pressure across the flow sensor. Such a flow is strictly internal and should not be reported to customer.
Another example of aspects of the thermal flow sensor signal that should not be reported to the operator are possible sharp spikes of the fluid flow through the sensor at the moment when valve is moving a long distance very quickly. Usually these long and fast valve movements occur when a non-zero-flow set point is given after a zero-flow set point, and vice versa.
Although there have been attempts to filter the indicated flow that is provided to an operator (e.g., to reduce false alarms), some of the prior approaches rely upon sophisticated algorithms that have proven to be unworkable in many instances because they utilize empirical parameters that may substantially vary from actual operating conditions.
Many of the existing filters are low pass filters (“LPF”) with an adjustable time constant. A disadvantage of this type of filter is the manner the time constant is adjusted in an attempt to provide an acceptable indicated flow output. Usually this adjustment of the data is done based on an allowed flow deviation off a baseline (set point). When measured flow is within a specified threshold off the set point, the filter time constant is high, so that the noise reduction is high. And if, for any reason, flow readings move out of specified range, the filter time constant momentarily decreases, immediately producing a noisy indicated flow. As the flow returns back to the set point, the filter time constant slowly increases; thus the noise slowly decreases. From a user's point of view, such behavior of the indicated flow makes it appear as though there are instantaneous “flow oscillations,” or an instability, but these aberrations are just a result of inadequate filtering of the indicated flow and do not represent actual flow.
Some other algorithms simply hide the measured flow while the measured values are out of an allowed range (typically around the flow set point), and these algorithms typically generate an indicated flow output that reports constant flow until the actual flow (as measured) returns back to the set point. But if the flow deviation lasts too long, the filter starts showing actual flow immediately, which produces a flow spike in the indicated flow that does not really exist.
Accordingly, a need exists for a method and/or apparatus to provide new and innovative features that address the shortfalls of present methodologies in generating an indicated flow of fluid flow conditions.